Conventionally, there is known a decoloring device that performs a decoloring process that heats a sheet, on which an image is printed by a decolorable coloring agent, to thereby decolor the image and that puts the sheet in a reusable state. In the decoloring device, there are cases where an undecolored residue remain on a sheet; however, in regard to a sheet on which a typical document is printed, even if a minute undecolored residue is present, it may be reused. However, in regard to a sheet on which a specific document such as a confidential document is printed, if the undecolored residue remain, it may not be reused.
In the conventional decoloring device, there is a problem that if the undecolored residue occurs on a sheet on which a specific document such as a confidential document is printed, the sheet cannot actually be reused, but the sheet may be regarded as a reusable sheet.